79 lines
1.8 KiB
Markdown
79 lines
1.8 KiB
Markdown
---
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title: Subsetting Data in R
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---
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## What is subsetting?
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Subsetting is the selection or extraction of specific parts of larger data. We can subset on various kinds of data objects: vectors, lists, and data frames.
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## Subsetting operators
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There are three subsetting operators: `[`, `[[` and `$`.
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`[[` is similar to `[`, except it can only return a single value and it allows you to pull elements out of a list.
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`$` is a useful shorthand for `[[` combined with character subsetting.
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You need `[[` when working with lists. This is because when `[` is applied to a list, it always returns a list; it never gives you the contents of the list.
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The following are the examples of subsetting of various `R` objects:
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**1. Vectors**
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```r
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x <- c(2.1, 4.2, 3.3, 5.4)
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x[c(3, 1)] # Subsetting using positive integers: return elements at the specified positions.
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## [1] 3.3 2.1
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x[-c(3, 1)] # Subsetting using positive integers: return elements at the specified positions.
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## [1] 4.2 5.4
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x[c(TRUE, TRUE, FALSE, FALSE)] # # Subsetting using logical vectors.
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## [1] 2.1 4.2
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```
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**2. Lists**
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```r
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a <- matrix(1:9, nrow = 3)
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colnames(a) <- c("A", "B", "C")
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a[1:2, ]
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## A B C
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## [1,] 1 4 7
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## [2,] 2 5 8
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```
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**3. Data Frames**
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```r
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df <- data.frame(x = 1:3, y = 3:1, z = letters[1:3])
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df[df$x == 2, ]
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## x y z
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## 2 2 2 b
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df[c(1, 3), ]
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## x y z
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## 1 1 3 a
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## 3 3 1 c
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```
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To get content of a list use `[[` operator like:
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```r
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a <- list(a = 1, b = 2)
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a[[1]]
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## [1] 1
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a[["a"]]
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## [1] 1
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```
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## Resources
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* [Quick-R](https://www.statmethods.net/management/subset.html)
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* [R Documentation](https://www.rdocumentation.org/packages/base/versions/3.5.1/topics/subset)
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* [R Bloggers](https://www.r-bloggers.com/5-ways-to-subset-a-data-frame-in-r/)
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* [Advanced R](http://adv-r.had.co.nz/Subsetting.html)
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